1. Field of the Invention
The present invention relates to a novel method of preparing levoglucosenone (1,6-anhydro-3,4-dideoxy-.beta.-D-glycero-hex-3-enopyranos -2-ulose).
2. Description of the Prior Art
Levoglucosenone is known as a sugar derivative. This is an optically active organic compound in which all carbon atoms are different environments and which has easily modifiable functional groups. Having these characteristics, levoglucosenone is a very useful compound for organic synthesis. As has been proved, it can be utilized as the starting material in the synthesis of 2,3-dideoxyribose which is useful as a medicine or the like. Also, levoglucosenone is used as the starting material for synthesizing optically active compounds, as is disclosed in Carbohydr. Res., 61, 519 (1978); 71, 169 (1979); 114, 71 (1983).
As has hitherto been known, levoglucosenone can be preparated by thermally decomposing cellulose. (See Carbohydr. Res., 61, 519 (1978); 67, 433 (1978), J.C.S. Perkin Trans. 1, 49 (1988), and U.S. Pat. No. 3,926,947 etc.). Also, it can be preparated by thermally decomposing cotton, chitin, glucose, or the like, as is disclosed in J. Macromol, Sci. Chem., A21, 385 (1984), Angew.Chem., 90, 802 (1978), and Carbohydr. Res., 46, 149 (1976).
These conventional methods have following disadvantages. First, they can provide but a low yield of 5% or less. Second, they makes a great amount of a reaction residue as by-product. Third, they require high thermal decomposition temperatures of 300.degree. C. or more. Fourth, it is difficult to heat the material uniformly since the material is a solid such as cellulose. In view of these disadvantages, the conventional methods are regarded as not suitable for preparation of levoglucosenone on an industrial scale.
On the other hand, as is known in the art, 1,6-anhydro-.beta.-D-galactopyranose identified by formula (4) can be prepared easily with a high yield from D-galactose which is abundant in nature and identified by formula (1), by the method represented by the following scheme I, as is disclosed in Berichte, 687 (1929), J. Am. Chem. Soc., 64, 2435 (1942); Chemistry of the Carbohydrates. Acad. Press, N.Y., p. 214 (1948); and Chem. Ind., 1637 (1967) etc.: ##STR1##
Also, it is known that 1,6-anhydro-3,4-dideoxy derivative identified by formula (6') can be obtained starting from 1,6-anhydro-.beta.-D-galactopyranose (4). In this method, as is represented by scheme II, 2-mesylorthoester identified by formula (5') which has been prepared from the starting material (4) is thermally decomposed at 170.degree. C. (see Natural and Applied Science Bulletin, Vol. 32, 1-4 (1980)). This method, however, requires five reactions steps, 1,6-anhydro-3,4-dideoxy derivative (6') which is the final product, is obtained from the starting material (4). Moreover, since the 2-mesylorthoester (5') is thermally decomposed at high temperature of 170.degree. C., the 1,6-anhydro-3,4-dideoxy derivative (6') is prepared, but at an extremely low yield of about 4%. ##STR2##
As is pointed out in Natural and Applied Science Bulletin, it is difficult to obtain the ortho ester (5') directly from the starting material (4), and then to convert this ortho ester (5') to 1,6-anydro-3,4-dideoxy derivative (6').